His concrete political positions certainly aren't always convincing. I know that Christopher Wise has some good criticism of his ambiguous statements about Israel, for instance. But I don't see how this vitiates his entire body of work. His primary concern is the history of Western philosophy and I always felt that there was more than a hint of Marx in the way he criticizes texts immanently with a focus on binary opposites. Now, you might say that it is no longer necessary to read philosophy at all because the science of dialectical materialism has made it obsolete, but that is not the position of Marx, Lenin or Mao. All of them take elements of their thought from Hegel because they have read him critically. Why should we not do the same? And in a way, basically everything Derrida wrote concerns the problem of reading. As far as I know, there is no dialectical materialist method of reading, so it's not like there's an obvious substitute for his work.

Regarding his effect on the intellectual esteem of Marxism in his time, I find it difficult to make a judgement. It seems to me that after 1968, there was no longer any possibility of worthwhile Marxist praxis in the West (for the time being at least). So I'd say there's a lot of blame to go around for the weakness of the Marxist left in Europe in the past decades, and I do not think that French intellectuals are a major factor here. If anything, the whole intellectual environment of "continental philosophy" seems more amenable to Marxist thought than Anglo analytic philosophy, which is the only alternative in Western universities. Maybe Specters of Marx didn't do anything for the Communist movement, but it did help a bit to make Marx seem intellectually respectable again after the decades of the Cold War.

In any case, Derrida's thinking about text and reading seems irreplaceable to me. Literature has always been a difficult topic for Marxism (the great names have almost nothing to say about it), so I think a kind of literary theory that is actually aware of the problems and history of philosophy instead of shunting that off to another discipline seems worthwhile, and I don't see how you get that without Derrida or thinkers like him.